surface acoustic wave devices are the devices utilizing surface acoustic waves, whose elastic energy is transmitted convergently on the surface of a solid. They are applied to intermediate frequency filters of television sets, since compact devices of stable performance are able to be fabricated. In general, a surface acoustic wave device has a piezoelectric plate and interdigital transducers (IDT) formed on the piezoelectric plate. It is excited by applying alternating voltage to the inter digital transducers. The piezoelectric material for the plate is a bulk monocrystal of quartz, LiNbO.sub.3 or LiTaO.sub.3, or a ZnO film grown on a substrate by a vapor phase method.
In general, the active frequency f of a surface acoustic wave device is determined by f=v/.lambda., where .lambda. is a wavelength and v is a propagation velocity of surface acoustic wave. The wavelength .lambda. is determined by the spacing frequency of inter digital transducers as shown in FIG. 1. The propagation velocity v is 3500 m/sec to 4000 m/sec for a LiNbO.sub.3 monocrystalline piezoelectric plate, and 3300 m/sec to 3400 m/sec for a LiTaO.sub.3 monocrystalline piezoelectric plate. It is at most 3000 m/sec for a glass substrate coated with a piezoelectric ZnO film.
The active frequency f can be heightened either by raising the propagation velocity v or by decreasing the wavelength .lambda.. However, the propagation velocity is restricted by the material property of the piezoelectric plate. The wavelength .lambda.; spacing frequency of interdigital transducers, is also restricted by a lower limit determined by the current fine processing technology. For example, the current optical lithography technology cannot fabricate a line/groove structure with a width less than 0.8 .mu.m. Electron beam exposure technology can depict submicron patterns. However, the narrower the line width becomes, the lower the yield. By these reasons, the maximum frequency of current surface acoustic wave devices in practical use is 900 MHz. Recently, satellite communication and mobile radio communication are likely to use higher frequencies for carrier waves of communication. Therefore, new surface acoustic wave devices which work at a frequency in the GHz band have been required. No surface acoustic wave devices operable in the GHz band have been produced to date.
In the case of surface acoustic wave devices utilizing a piezoelectric film grown on a substrate, plural surface acoustic waves (called zeroth mode, first mode, second mode . . . , according to the order of increasing velocity) are excited, if the sound velocity of the substrate is higher than that of the piezoelectric film. The velocities of all modes depend also on the substrate in the case. The higher the sound velocity of the substrate, the higher the velocities of all modes of the surface acoustic wave of the device including a piezoelectric plate and the substrate are. Roughly speaking, the surface acoustic wave velocity increases in proportion to the sound velocity of the substrate. However, the surface acoustic wave velocity is different from the sound wave velocity. These two velocities must rigorously be distinguished.
In order to heighten the surface acoustic wave velocity, an improved device with a ZnO piezoelectric film grown on a sapphire substrate was proposed by Japanese Patent Laying Open No. 50-154088. Sapphire was chosen as a material for the substrate, because sapphire is favored with high sound velocities; 6000 m/sec for traverse waves and 12000 m/sec for longitudinal waves. It was reported that the surface acoustic wave velocity was 5500 m/sec for the improved device (ZnO/sapphire).
Besides glass and sapphire, diamond is another promising material for a substrate of the surface acoustic wave devices, because of high rigidity. Diamond has the highest sound velocities of all materials. The sound velocity of traverse waves in diamond is 13000 m/sec. The sound velocity of the longitudinal waves is 16000 m/sec. No other material has such high sound velocities. Diamond-like carbon film have sound velocities as high as diamond, because diamond-like carbon film is endowed with as high a rigidity as diamond. If a diamond or diamond-like carbon film was employed to the material of the substrate, an improved surface acoustic wave device would have the surface acoustic wave more than 10000 m/sec. Japanese Patent Laying Opens No. 1-20714 and No. 1-62911 proposed such surface acoustic wave devices having the strata of a diamond substrate and a piezoelectric film.
Besides the simple surface acoustic wave devices with a piezoelectric plate and electrodes, other devices making use of interaction of piezoelectric material with semiconductor have been proposed, i.e., surface acoustic wave convolvers, surface acoustic wave phase-shifters and surface acoustic wave amplifiers.
(a) Surface Acoustic Wave Phase-Shifter
A surface acoustic wave phase-shifter shown in FIG. 2 has been proposed. The phase-shifter has a silicon (Si) semiconductor substrate (21), a silicon dioxide (SiO.sub.2) film (22) deposited on the Si substrate (21), lower electrodes (6) formed on ends of the SiO.sub.2 film (22), a zinc oxide (ZnO) piezoelectric film (30) deposited on the SiO.sub.2 film (22) and the electrodes (6), interdigital transducers (4) formed on ends of the ZnO film (30), a gate electrode (5) formed in the middle of the ZnO film (30), and an ohmic electrode (7) formed on a bottom of the Si substrate (21).
This is a simple, stable surface acoustic wave phase-shifter of monolithic structure. Attention is paid to the application of the phase-shifter to a voltage-controlled oscillator (VCO).
(b) Surface Acoustic Wave Amplifier
A surface acoustic wave amplifier shown in FIG. 3 has been proposed. The amplifier has a lithium niobate (LiNbO.sub.3) piezoelectric monocrystalline substrate (23), a silicon dioxide (SiO.sub.2) film (24) deposited on the LiNbO.sub.3 substrate (23), an indium antimony (InSb) semiconductor film (25) grown on the SiO.sub.2 film (24) , and a SiO.sub.2 film (26) deposited on the InSb film (25).
(c) Surface Acoustic Wave Convolver
A surface acoustic wave convolver shown in FIG. 4 has been proposed. It has a LiNbO.sub.3 piezoelectric monocrystalline substrate (27), a SiO.sub.2 insulating film (28) deposited on the LiNbO.sub.3 substrate (27), interdigital transducers (18) formed on ends of the LiNbO.sub.3 substrate (27), a Si semiconductor film (29) grown on the SiO.sub.2 film (28), an upper electrode (19) deposited on the Si film (29) and a lower electrode (20) deposited on the bottom of the LINbO.sub.3 substrate (27).
FIG. 5 shows another surface acoustic wave convolver using a ZnO film as a piezoelectric vibrator. It has a Si semiconductor substrate (31), a SiO.sub.2 insulating film (32) deposited on the Si substrate (31), a zinc oxide (ZnO) piezoelectric film (33) grown on the SiO.sub.2 film (32), interdigital transducers (34) formed on ends of the ZnO film (33) and an output electrode (35) formed on the ZnO film (33). Another output electrode as a ground (36) is formed on the bottom of the Si substrate (31). It has been explained so far that surface acoustic wave phase-shifters, surface acoustic wave amplifier and surface acoustic wave convolvers were proposed as surface acoustic wave devices.
The surface acoustic wave devices works at the frequency (v/.lambda.) determined by the surface acoustic wave velocity (v) and the spacing frequency (.lambda.) of inter digital transducers. In order to raise the performance of the device, the working frequency must be heightened. Heightening the working frequency requires either smaller electrode-distance and electrode-width, or larger surface acoustic wave velocity. But it is difficult to fabricate microscopic interdigital transducers with high yield. As mentioned before, the current optical lithography cannot make electrodes of a line width and a spacing narrower than 0.8 .mu.m with sufficiently high yield. Thus, raising surface acoustic wave velocity is essential to obtain surface acoustic wave devices of high quality in high frequency region (more than 1 GHz).
A purpose of this invention is to provide a surface acoustic wave device with higher surface acoustic wave velocity than the prior art. Another purpose of the invention is to provide a surface acoustic wave device with comparatively wide electrodes and wide spacings therebetween, which can alleviate the difficulty of producing interdigital transducers.